1. Field of the Invention
The invention relates in general to the construction of measuring probe and in particular to a new and useful measuring probe for the polarographic determination of gas partial pressures in aqueous, more particularly biologic, medium, which includes at least a part made up of thin films, and where a diffusion film membrane applies directly on at least the polarographic surface.
2. Description of the Prior Art
Such probes, which are used in particular for pO.sub.2 determination, are described in DT-OS 2,501,399, W. Siu (Master Thesis, University of Toronto, Institute of Biomedical engineering) "Design optimization and fabrication of integrated circuit multicathode oxygen electrodes" and H. Baumgartl and D. W. Lubbers, article "Platinum needle electrode for polarographic measurement of oxygen and hydrogen" in "Oxygen Supply", published by Urban and Schwarzenberg, 1973, p. 130.
In these probe constructions, a membrane must be arranged between the polarographic electrode (the cathode in the case of pO.sub.2 measuring probes) and the medium in which the gas pressure is to be determined, at least when such probes are to be used in flowing liquids or, for example, liquids containing proteins, such as blood and other body fluids. It is the function of the membrane to let, apart from the gas to be measured, only those substances get from the medium to the polarographic surface, which do not influence the polarographic reaction, also, as an impediment of diffusion for the gas to be measured. It is the function of the membrane to lower the gas consumption of the polarographing surface to the extent that in the medium before the membrane only a negligibly small gas diffusion field builds up.
The gas pressure gradient forming, before any polarographing surface is displaced out of the surrounding medium into the membrane, almost completely when the diffusibility for the gas to be measured is more than one order of magnitude less in the membrane than in the surrounding medium. The quantity of gas reaching the polarographing surface, to which the polarographic current, or the measuring signal, is proportional, is then determined only by the gas pressure gradient above the membrane. The gas pressure gradient above the membrane corresponds to the gas pressure in the medium, because zero gas pressure prevails on the polarographing surface. Such probes, therefore, are free of stir effect, i.e. they indicate the gas pressure in the medium independently of flows in the medium.
In oscillographic polarography (rhythmic conduction of the polarographic process), the membrane has in principle the same functions, even though in such a use a stationary diffusion field does not build up in the membrane.
A comprehensive analysis of the gas diffusion in such membranes has been given for example by C. Schneiderman in the dissertation "Arterial wall oxygen transport system: Computer simulation and experimental study, including a theoretical analysis of various tissue oxygen microelectrodes" at the University of Evanston, Illinois, 1975.
Membranes on polarographing surfaces are subjected to considerable chemical loads. In oxygen polarography, for example, H.sub.2 O.sub.2 and hydroxyl ions occur as reaction products of the oxygen at the polarographing surface. It must be possible to diffuse these reaction products away through the membrane. In addition, this reaction requires water, which it must be possible to diffuse in from the medium together with the oxygen to be measured.
As membrane materials organic polymers have been used heretofore, for example, PTFE, polystyrene, polyethylene, acrylates, and many other plastics. The major disadvantage in the use of such plastics is their insufficient chemical stability to water and the products occurring in polarography. In the practice, plastics do not reach a sufficiently stable swelling equilibrium. Besides, the swelling equilibrium of the plastics depends on the concentration of resulting reaction products during the measurement. In the measuring practice, this results in the following problems:
1. Before it can be used for the measurement, the probe must be brought into swelling equilibrium with water for a prolonged time. This process takes hours or days, depending on the plastic and membrane thickness.
2. Before each measurement, calibration of the probe is necessary.
3. During prolonged measuring periods, the measurement indications of such probes drifts for a given gas pressure.
These disadvantages of plastic membranes can be reduced by making them relatively thick, in order to reduce the gas consumption and hence damage to the membrane by reaction products, and possibly by enriching the membrane with swelling agents in order to reach a relatively stable swelling state sooner. Also tests have been made to improve the membrane properties by incorporation of functional groups into the polymers.